1. Field of the Invention
The present invention relates to a semiconductor device using an oxide semiconductor and a manufacturing method thereof. Note that in this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. As an example of such a semiconductor element, for example, a transistor (a thin film transistor and the like) can be given. In addition, a semiconductor device also refers to a display device such as a liquid crystal display device.
2. Description of the Related Art
There is a trend in a display device using a transistor (e.g., a liquid crystal panel and an organic EL panel) toward a larger screen. As the screen size becomes larger, in the case of a display device using an active element such as a transistor, a voltage applied to an element varies depending on the position of a wiring which is connected to the element due to wiring resistance, which cause a problem of deterioration of display quality such as display unevenness and a defect in grayscale.
Conventionally, an aluminum film has been widely used as a material used for the wiring, the signal line, or the like; moreover, research and development of using a copper film as a material is extensively conducted to further reduce resistance. However, a copper film is disadvantageous in that adhesion thereof to a base film is low and that characteristics of a transistor easily deteriorate due to diffusion of copper element in the copper film into a semiconductor layer of the transistor. Note that a silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor, and as another material, an oxide semiconductor has attracted attention (e.g., Patent Documents 1 and 2).
In a transistor using a silicon-based semiconductor material, research and development on a structure in which a copper film is used for a wiring, a signal line, or the like and copper element of the copper film are not diffused to a semiconductor layer have been extensively conducted. However, there has been a problem in that the structure and a method for manufacturing it are not suitable yet for a transistor using an oxide semiconductor film.
Further, in the transistor using an oxide semiconductor film in which a copper film is used for a wiring, a signal line, or the like and a barrier film is used to suppress diffusion of copper element of the copper film, when the structure in which copper element of the copper film are not diffused to the oxide semiconductor film is employed in order not to affect electric characteristics of the oxide semiconductor film, there has been a problem in that the number of masks and the manufacturing cost were increased.